Bipolar transistors are currently used as sensor elements of imaging arrays. Each transistor can be used both as an integrating photosensor and as a select device. The phototransistor sensor produces an output (photo)current as a result of absorbing photons, with the output current being proportional to the intensity of light incident on the sensor. Photons absorbed in the area of a phototransistor collector-base or emitter-base junction produce electron-hole pairs that are collected by a nearby p-n junction. Minority carriers collected by either junction act as a base current and are multiplied by the transistor gain to produce the collector current. The emitter current is the sum of the base current and collector current, and is usually used as the output of the sensor.
FIG. 1 is a cross-sectional view showing the structure of a prior art bipolar phototransistor 10 which can be used as a sensing element in an imaging array. Phototransistor 10 is fabricated in n-well 12 which is formed in a semiconductor substrate (not shown) using a standard n-well CMOS fabrication process, to which is added p-type base region diffusion and emitter definition steps. N-well 12 serves as a common collector for the phototransistors in the photosensor array and is biased at voltage V.sub.cc. P-type base region 14 of phototransistor 10 is formed by implanting the appropriate dopant species into n-well 12 through openings in field oxide layer (FOX) 16. Gate oxide layer 18 acts as the dielectric for a capacitor which couples base region 14 of phototransistor 10 to row select line 20 formed from an n+ polysilicon layer. The plates of the capacitor are p-type base region 14 and select line 20.
In an array of such sensing elements, the base terminals of all the phototransistors in a row are capacitively coupled to a common row select line 20. An n+ region 22 formed in p-type base region 14 serves as the emitter of phototransistor 10. N+ region 22 is typically formed by implanting an n-type dopant through an aperture in select line region 20 and gate oxide layer 18. Oxide layer 24 is deposited over select line region 20. Metal contact 26 is formed in oxide layer 24 using conventional semiconductor processing methods and serves as a contact between emitter 22 of phototransistor 10 and a column sense line. Emitter 22 serves as the output node for the integrating photosensor of FIG. 1, with the output nodes of all of the sensors in a column being connected to a common column sense line. It is noted that FIG. 1 shows the structure of an npn type phototransistor and that a corresponding pnp type phototransistor may be formed by interchanging the n-type and p-type regions and reversing the polarities of the associated voltages.
As noted, phototransistor 10 is fabricated by modifying a standard CMOS process to include the step of forming p-type base region 14. This step is performed after field oxide formation and nitride strip, or a similar step in the standard process. Gate oxide layer 18 and row select line 20 are formed during the processing steps which are used to form the gate oxide and polysilicon gates of the MOS devices. Emitter 22 is formed during the standard n+ source/drain implant step, and the column sense line is formed during the contact and metallization steps of the fabrication process.
During the operation of phototransistor 10, select line 20 is held at a fixed voltage which is chosen to reverse bias the base-emitter junction of phototransistor 10. As image photons impact the phototransistor, electrons and holes are generated. The photo-generated electrons are swept into n-well 12 and removed through the collector voltage line (V.sub.CC). The photo-generated holes accumulate in p-type base region 14 and produce an increase in the base potential. The photocurrent generated integrates on the capacitor which is formed as part of the structure of phototransistor 10 of FIG. 1. This corresponds to an image storage operation. When it is desired to read out the stored image contained in the imaging element represented by phototransistor 10, select line 20 is brought to a high value, thereby forward biasing the base-emitter junction of the transistor. In this situation, the integrated photocurrent, multiplied by the current gain of phototransistor 10 flows from emitter region 22 to the column sense line by means of metal contact 26. An integrating sense amplifier connected to the column sense line is used to sense the current produced by each of the sensor elements contained in the column.
The phototransistor shown in FIG. 1 has several advantages over other types of imaging elements: 1) the structure may be implemented in a smaller size than corresponding CCD and CMOS compatible elements; and 2) the fabrication process is CMOS compatible, having one additional mask for the base implant. The polysilicon layer (row select line region) to base capacitor dielectric is the gate oxide layer, and the emitter region is self-aligned by the n+ implant.
However, the phototransistor of FIG. 1 does have disadvantages. When a strong image (intense light) is incident on the imaging element formed from the transistor, the base potential during the integration process increases rapidly due to the photo-generated holes, until the base-emitter junction is slightly forward biased. Further holes which are generated in the base region are injected into the emitter. This is termed the "overflow" problem, and it contributes to the column sense line signal as noise when other elements in the same column of the array are being sensed (the image is being read). This produces a vertical noise pattern on a bright image spot.
A second disadvantage is that those imaging elements exposed to a strong image are difficult to reset back to the same reverse bias level for purposes of image storage (photo-generated charge integration). The residue charge requires a relatively long time period (approximately 100 ms) to be recombined into the base of the phototransistor. This produces an image lag or "blooming" effect in which a bright tail is associated with a moving bright image spot.
Both the overflow and blooming problems noted contribute to a degradation of the image quality in the cases of strong images. This reduces the utility of such photosensors for obtaining images in situations of bright (intense) light.
What is desired is a design for a bipolar active pixel element which does not suffer from the overflow and blooming problems described.